In a large saucepan, melt the butter. Add the onion and cook over moderately high heat until tender, about 4 minutes. Add the orzo and cook, stirring frequently, until lightly toasted, about 3 minutes. Add the chicken broth and 1 teaspoon of salt and bring to a simmer. Cover and cook over low heat until the orzo is tender, about 18 minutes. Fold in diced tomato and chopped basil, season with salt and pepper and serve.